Observation of Dirac hierarchy in three-dimensional acoustic topological insulators

TL;DR

This paper reports the first experimental observation of a Dirac hierarchy in 3D acoustic topological insulators, demonstrating how multifold Dirac cones lead to various topological states across different dimensions.

Contribution

It provides the first experimental evidence of Dirac hierarchy in 3D acoustic TIs, linking multifold Dirac cones to topological states from surface to corner levels.

Findings

Lifting of multifold Dirac cones induces 2D surface states in first-order TIs.

Lifting of multifold Dirac cones induces 1D hinge states in second-order TIs.

Lifting of multifold Dirac cones induces 0D corner states in third-order TIs.

Abstract

Dirac cones (DCs) play a pivotal role in various unique phenomena ranging from massless electrons in graphene to robust surface states in topological insulators (TIs). Recent studies have theoretically revealed a full Dirac hierarchy comprising an eightfold bulk DC, a fourfold surface DC, and a twofold hinge DC, associated with a hierarchy of topological phases including first-order to third-order three-dimensional (3D) topological insulators, using the same 3D base lattice. Here, we report the first experimental observation of the Dirac hierarchy in 3D acoustic TIs. Using acoustic measurements, we unambiguously reveal that lifting of multifold DCs in each hierarchy can induce two-dimensional (2D) topological surface states with a fourfold DC in a first-order 3D TI, one-dimensional (1D) topological hinge states with a twofold DC in a second-order 3D TI, and zero-dimensional (0D) topological corner states in a third-order 3D TI. Our work not only expands the fundamental research scope of Dirac physics, but also opens up a new route for multidimensional robust wave manipulation.